Composition and method for inhibiting polymerization and polymer growth

ABSTRACT

A method for inhibiting the premature polymerization and the polymer growth of ethylenically unsaturated monomers is disclosed wherein the method comprises adding to said monomers an effective amount of at least one hydrogen donor or electron acceptor. In a preferred embodiment, the hydrogen donor or electron acceptor is used in combination with a stable nitroxyl free radical.

This is a division of U.S. application Ser. No. 09/580,343, filed May25, 2000, for which the benefit under Title 35, United States Code, §120 to U.S. Provisional Application No. 60/168,623, filed Dec. 3, 1999,entitled COMPOSITION AND METHOD FOR INHIBITING POLYMERIZATION ANDPOLYMER GROWTH has been claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the inhibition of polymerizationand polymer growth of ethylenically unsaturated monomers by means of theaddition thereto of hydrogen donors and/or electron acceptors, eitheralone or in combination with at least one stable nitroxide free radicalcompound.

2. Description of Related Art

Many ethylenically unsaturated monomers undesirably polymerize atvarious stages of their manufacture, processing, handling, storage, anduse. Polymerization, such as thermal polymerization, during theirpurification results in the loss of the monomer, i.e., a lower yield,and an increase in the viscosity of any tars that may be produced. Theprocessing and handling of the higher viscosity tars then requireshigher temperature and work (energy cost) to remove residual monomer.

Polymerization can also result in equipment fouling, especially in thecase of production of acrylic monomers. Such polymerization causes lossin production efficiency owing to the deposition of polymer in or on theequipment being used. These deposits must be removed from time to time,leading to additional loss in production of the monomer.

A wide variety of compounds has been proposed and used for inhibitinguncontrolled and undesired polymerization of ethylenically unsaturatedmonomers. However, many of these compounds have not been fullysatisfactory.

There are several mechanisms by which polymerization inhibitors work.One mode of action for polymerization inhibitors is for the inhibitingspecies to combine with the propagating polymer chain such that thepolymerization of that polymer chain stops, i.e., a terminationreaction. If such an inhibitor-terminated polymer chain is capable ofparticipating in a dynamic equilibrium between a dormant species (theinhibitor-terminated chain) and an active polymer chain, it would beconsidered a “living” or quasiliving polymer. For example, Ivan,Macromol. Symp. 88:201-215 (1994) describes quasiliving polymerizationas a polymerization in which “ . . . only a portion of chain ends areactive (propagating) and these are in equilibria with inactive (dormant,nonpropagating) chains . . . ” Shigemoto et al., Macromol. Rapid Commun.17:347-351 (1996) state, “Well-defined polymers can be prepared bycontrolled/“living” radical polymerization in the presence of relativelystable radicals. These systems employ the principle of dynamicequilibration between dormant species and growing radicals viareversible homolytic cleavage of a covalent bond in dormant species.”Further, Greszta et al., Macromolecules 29:7661-7670 (1996) state, “Thereversible homolytic cleavage of dormant species can be accomplished byeither thermal, photochemical, or catalytic activation. The mostsuccessful approaches are as follows: homolytic cleavage of alkoxyaminesand dithiocarbamates, use of various organometallic species, andcatalyzed atom transfer radical polymerization.” Such a “living” polymeris capable of increasing in molecular weight (growing) through itsreaction with additional monomer units of the same or different types ofpolymerizable monomers.

The method by which this “living” polymer grows is termed the “living”polymerization mechanism, and is depicted below. M − Inh ---> M* + *Inh(1) M* + *Inh ---> M − Inh (2) M* + M′ ---> M − M′* (3) M − M′* + *Inh---> M − M′ − Inh (4)Reactions (1) and (2) depict the dynamic equilibrium, with (2) being thetermination reaction. Reaction (3) depicts growth of the polymer chain.Reaction (4) depicts re-termination of the growing polymer chain withthe inhibiting species. The amount of growth over any period of time isdependent on the relative rate at which (2) occurs versus (3), as longas (1) occurs to some extent. The faster (2) is relative to (3), themore time is needed for significant growth of the polymer. Under theconditions in which inhibitors are normally used, the concentration ofthe inhibiting species should be sufficiently high to cause reaction (2)to be much faster than reaction (3), otherwise it would not be aneffective inhibiting system for commercial use. However, we haverealized that even at an effective inhibiting amount of the inhibitor,growth can still occur, given sufficient time and temperature.

There are at least two scenarios in which “living” polymer can remain ina monomer purification train for an excessive amount of time.

First, the use of recycle can significantly increase the amount of timethat the “living” polymer can remain in the purification train. Torecycle unused inhibitor that is left in the purification stream afterremoval of the monomer, a portion of the residual stream is added to afeed stream earlier in the purification train. This residual streamtypically contains inhibitor, small amounts of monomer, impurities inthe monomer stream that have been concentrated by the purificationprocess, and polymer formed during the production and purificationprocess. Recycling this polymer will allow it time to grow if it is“living” polymer and the conditions of the purification train allow the“living” polymerization mechanism to occur. If this polymer grows viathe “living” polymerization mechanism, excessive polymerization wouldcause loss in product yield, increased waste residues from the process,and potential plugging of equipment due to excessively high molecularweight polymer in the purification stream.

Second, occasionally, conditions in the plant/purification process canresult in the formation of polymer within the purification train that isnot dissolved by the monomer stream. If this polymer is caught in a deadspace, or if it attaches to the metal on the inside of the equipment, itwill not be washed out of the system. Thus, the polymer will remainwithin the system indefinitely (potentially for two or more years). Ifthis polymer grows via the “living” polymerization mechanism, it couldcoat the inside of the equipment, causing inefficient separation of themonomer stream components and/or insufficient heating of the stream toenable purification. Such a situation would cause loss in product yieldand could potentially cause an unscheduled shut-down of the plant inorder to clean out the undissolved polymer in the equipment. Such ashut-down results in loss of monomer production and additional expenseto clean out and dispose of the undissolved polymer.

It is significant that there has been no indication that previously usedinhibitors would lead to the formation of “living” polymer when used aspolymerization inhibitors. However, a newly utilized class ofinhibitors, the stable nitroxyl radicals, is known to allow this“living” polymerization mechanism to occur. These nitroxyl radicals arehighly efficient polymerization inhibitors under normal use, providingbetter performance than most other inhibitors on the market, but theirincapacity to prevent “living” polymerization has hindered their fullutilization. Accordingly, there is a need for compositions that can beused in a purification train, preferably in combination with nitroxylradicals, to prevent polymer growth that occurs via a “living”polymerization mechanism. Nitroxyl radicals are known to facilitatepolymerization via a “living” free radical process to give polymers ofnarrow polydispersity.

Georges et al., Macromolecules 26(11):2987-2988 (1993) synthesizednarrow molecular weight resins by a free-radical polymerization processwith polydispersities comparable to those that can be obtained byanionic polymerization processes and below the theoretical limitingpolydispersity of 1.5 for a conventional free-radical polymerizationprocess. The process comprised heating a mixture of monomer(s),free-radical initiator, and a stable free radical, e.g.,2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO).

Hawker et al., Macromolecules 29(16):5245-5254 (1996) prepared andevaluated a variety of initiating systems for the preparation ofmacromolecules by nitroxide-mediated “living” free radical procedures.The systems were divided into two classes, unimolecular initiators inwhich alkylated TEMPO derivatives dissociate to provide both theinitiating radical and the stable radical, and bimolecular systems inwhich a traditional free radical initiator, such as BPO or AIBN, is usedin conjunction with TEMPO. For the unimolecular initiators it was foundthat an α-methyl group is essential for “living” character, while avariety of substituents could be placed on the phenyl ring or theβ-carbon atom without affecting the efficiency of the unimolecularinitiator. It was found that the rate of polymerization is approximatelythe same for both the unimolecular and corresponding bimolecularsystems; however, the unimolecular initiators afforded better controlover molecular weight and polydispersity.

The inventors are unaware of any art on the use of compounds to preventpolymer growth that occurs via a “living” polymerization mechanism sincethis growth phenomenon is not known to have previously been observed.Hindered nitroxyl compounds are known to be very active inhibitors offree radical polymerizations of unsaturated monomers such as styrene,acrylic acid, methacrylic acid, and the like. Nitrophenols,nitrosophenols, phenylenediamines (PDA's), hydroxylamines, quinones andhydroquinones are also known to have a similar capacity.

U.S. Pat. No. 2,304,728 discloses that a vinyl aromatic compound mayeffectively be stabilized against polymerization by dissolving therein amonohydric halo-nitrophenol having the general formula:

wherein one X represents a halogen and the other X represents a memberof the group consisting of hydrogen and halogen and nitro substituents.

U.S. Pat. No. 3,163,677 discloses a process for the preparation ofN,N,O-trisubstituted hydroxylamines and N,N-disubstituted nitroxides ofthe formulae:

wherein R₁, R₂, and R₃ are each an alkyl radical having 1 to 15 carbonatoms. (As used herein, the designation N—O* denotes a stable freeradical wherein the asterisk is an unpaired electron.) TheN,N,O-trisubstituted hydroxylamines can be used to make theN,N-disubstituted nitroxides, which are stable free radicals and aresaid to be useful as polymerization inhibitors.

U.S. Pat. No. 3,334,103 discloses that nitroxides can be prepared fromthe corresponding heterocyclic amine wherein the nitrogen atom of thenitroxide group is attached to other than a tertiary carbon of analiphatic group (i.e., the nitrogen atom forms a part of a heterocyclicnucleus). These nitroxides are said to have useful properties similar tothose described for the N,N-disubstituted nitroxides of U.S. Pat. No.3,163,677.

U.S. Pat. No. 3,372,182 discloses that a great variety ofN,N-disubstituted, stable, free radical nitroxides not otherwise readilyavailable can be prepared by a simple and convenient process thatcomprises pyrolyzing in an inert reaction medium virtually anyhydroxylamine that is susceptible to cleavage of the O—C bond, e.g.,tri-t-butylhydroxylamine.

U.S. Pat. No. 3,422,144 discloses stable, free radical nitroxides of theformula:

wherein R is selected from the group consisting of tertiary alkyl, aryl,alkaryl, haloaryl, carboxyaryl, alkoxyaryl, alkylthioaryl, pyridyl, anddialkylaminoaryl, and R′ is tertiary alkyl. These nitroxides are said tobe useful as traps for reactive free radicals both in the counting offree radicals and for inhibiting oxidation and free radicalpolymerization.

U.S. Pat. No. 3,494,930 discloses free radicals of the nitroxide typefor use as initiators of free radical reactions, collectors of freeradicals, polymerization inhibitors or antioxidants. They areconstituted by nitrogenous bicyclic compounds in which one of thebridges comprises solely the nitroxide radical group and, in particular,by aza-9-bicyclo (3,3,1) nonanone-3-oxyl-9, and by aza-9-bicyclo (3,3,1)nonane oxyl-9.

U.S. Pat. No. 3,873,564 discloses compounds and a method for assayingenzymes by adding to a medium containing an enzyme a stable free radicalcompound having a stable free radical functionality which, whensubjected to an enzyme-catalyzed reaction, changes the environment ofthe free radical functionality. By following the change in the electronspin resonance spectrum as affected by the change in environment, thetype of enzyme and the activity of the enzyme can be determined. Thecompounds found useful are normally stable nitroxide radicals with anenzyme labile functionality. Other compounds include two cyclicnitroxide containing rings joined by a chain having an enzyme labilefunctionality.

U.S. Pat. No. 3,966,711 teaches that 2,2,7,7-tetraalkyl- and2,7-dispiroalkylene-5-oxo-1,4-diazacycloheptanes substituted in the4-position by mono- or tetravalent radicals are powerfullight-stabilizers for organic polymers. They are said to possess highercompatibility than their 4-unsubstituted homologues, from which they canbe synthesized by reactions known for N-alkylation. Preferredsubstituents in the 4-position are alkyl, alkylene, alkenyl, aralkyl,and esteralkyl groups. The 1-nitroxyls derived from the imidazolidinesby oxidation with hydrogen peroxide or percarboxylic acids are also saidto be good light stabilizers.

U.S. Pat. No. 4,105,506 discloses a process for the distillation ofreadily polymerizable vinyl aromatic compounds and a polymerizationinhibitor therefor. The process comprises subjecting a vinyl aromaticcompound to elevated temperatures in a distillation system in thepresence of a polymerization inhibitor comprising 2,6-dinitro-p-cresol.

U.S. Pat. Nos. 4,252,615 and 4,469,558 disclose a process for thedistillation of readily polymerizable vinyl aromatic compounds and apolymerization inhibitor therefor. The process comprises subjecting avinyl aromatic compound to elevated temperatures in a distillationsystem in the presence of a polymerization inhibitor comprising2,6-dinitro-p-cresol. Also disclosed is a distillation method andapparatus for use with this inhibitor.

U.S. Pat. No. 4,434,307 discloses the stabilization of vinyl aromaticcompounds against undesired polymerization by adding to the vinylaromatic compounds small amounts of at least one N,N-diarylhydroxylamineand at least one mono- or ditertiary alkyl catechol and/or at least onemono-or ditertiary alkylhydroquinone.

U.S. Pat. No. 4,439,278 discloses an improvement in methods forpreparing and processing ethylenically unsaturated aromatic monomer. Theimprovement comprises employing 3,5-dinitrosalicylic acid or aderivative or isomer thereof as a process inhibitor. The processinhibitor is present in a concentration of about 50 to 3000 ppm,preferably about 250 to 2,000 ppm, and most preferably about 500 to1,000 ppm.

U.S. Pat. No. 4,466,904 discloses a compound and a process for utilizingthe compound to prevent the polymerization of vinyl aromatic compounds,such as styrene, during heating. The compound includes effective amountsof phenothiazine, 4-tert-butylcatechol and 2,6-dinitro-p-cresolrespectively, as a polymerization inhibitor system in the presence ofoxygen resulting in a less viscous polymer tar and in the effectiveinhibition of polymerization to temperatures as high as 150° C.

U.S. Pat. Nos. 4,466,905 and 4,468,343 disclose a compound and a processfor utilizing the compound to prevent the polymerization of vinylaromatic compounds, such as styrene, during heating. The compositionincludes effective amounts of 2,6-dinitro-p-cresol and either aphenylenediamine or 4-tert-butylcatechol respectively, to act as apolymerization co-inhibitor system in the presence of oxygen.

U.S. Pat. No. 4,480,116 discloses an improvement in methods forpreparing and processing readily polymerizable acrylate monomers. Theimprovement comprises employing phenyl-para-benzoquinone,2,5-di-phenyl-para-benzoquinone, and mixtures thereof as processinhibitors. The process inhibitors are present in a concentration ofabout 50 to 3000 ppm, preferably about 250 to 2000 ppm, and mostpreferably about 500 ppm.

U.S. Pat. No. 4,558,169 discloses a process for preparing vinyltoluenecomprising passing ethyltoluene through a dehydrogenation zone to formvaporous crude vinyltoluene, adding from about 50 to about 100 parts permillion by weight of a polymerization inhibitor such as a nitratedphenol to the vaporous crude vinyltoluene at a temperature between about200° and about 300° C., condensing the vaporous crude vinyltoluene,maintaining the pH of the aqueous phase of the condensed crudevinyltoluene at a value between about 5.5 and about 6.5 sufficient tomaintain the inhibitor in the organic phase of the condensed crudevinyltoluene, adding a second portion of polymerization inhibitor to thecondensed crude vinyltoluene until the inhibitor concentration totalsabout 500 parts per million by weight relative to the vinyltoluenecontent of the crude vinyltoluene, filtering the condensed crudevinyltoluene to remove seed polymer, and distilling the condensed crudevinyltoluene to recover substantially pure vinyltoluene; and apparatusfor carrying out said method.

U.S. Pat. No. 4,665,185 discloses a process for the efficientpreparation of nitroxyls of sterically hindered amines by the oxidationof the amine using a hydroperoxide in the presence of a small amount ofa metal ion catalyst, at moderate temperature for a short period oftime, to give the nitroxyl in high yield and purity.

U.S. Pat. No. 4,692,544 discloses certain substituted diaryl amines thatare used to inhibit the polymerization of ethylenically unsaturatedmonomers; for example, unsaturated carboxylic acids and derivativesthereof.

U.S. Pat. No. 4,720,566 discloses compositions and methods of inhibitingacrylonitrile polymerization, particularly in quench columns of systemsproducing acrylonitrile, comprising adding to the acrylonitrile aneffective amount for the purpose of (a) a hydroxylamine having theformula

wherein R and R′ are the same or different and are hydrogen, alkyl,aryl, alkaryl or aralkyl groups, and (b) a para-phenylenediamine orderivative thereof having at least one N—H group. Preferably thephenylenediamine is a para-phenylenediamine having the formula

wherein R¹, R², R³ and R⁴ are the same or different and are hydrogen,alkyl, aryl, alkaryl, or aralkyl groups with the proviso that at leastone of R¹, R², R³ or R⁴ is hydrogen.

U.S. Pat. No. 4,774,374 discloses a vinyl aromatic compositionstabilized against polymerization comprising (a) a vinyl aromaticcompound and (b) an effective amount of a stabilizer system in which theactive ingredient consists essentially of an oxygenated species formedby the reaction of oxygen and an N-aryl-N′-alkyl-p-phenylenediamine.

U.S. Pat. No. 4,797,504 discloses compositions and methods of inhibitingacrylate monomer polymerization at elevated temperatures comprisingadding to the acrylate monomer an effective amount for the purpose of(a) a hydroxylamine having the formula

wherein R and R′ are the same or different and are hydrogen, alkyl,aryl, alkaryl or aralkyl groups, and (b) a para-phenylenediamine orderivative thereof having at least one N—H group. Preferably thephenylenediamine is a para-phenylenediamine having the formula

wherein R¹, R², R³ and R⁴ are the same or different and are hydrogen,alkyl, aryl, alkaryl, or aralkyl groups with the proviso that at leastone of R¹, R², R³ or R⁴ is hydrogen.

U.S. Pat. No. 4,912,247 discloses a composition and method of use forinhibiting the polymerization of acrylate esters during elevatedtemperature processing and during storage and handling thereafter. Itcomprises the combination of a Mannich reaction product, which isprepared from a substituted phenol, an aldehyde and ethylenediamine, andeither phenylenediamine or derivatives thereof and/or phenothiazine orderivatives thereof.

U.S. Pat. No. 4,929,778 discloses methods and compositions forinhibiting the polymerization of styrene monomer during elevatedtemperature processing thereof or during storage or shipment of styrenecontaining product. The compositions comprise a combination of (a) aphenylenediamine compound having at least one N—H bond and (b) ahindered phenol compound. The methods comprise adding from 1-10,000 ppmof the combination to the styrene medium, per one million parts ofstyrene.

U.S. Pat. No. 5,128,022 discloses methods and compositions forinhibiting the formation of polymers in petroleum or petrochemicalprocesses that subsequently foul heat transfer surfaces. Thecompositions comprise a combination ofN-Phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine (PDA) and an organicacid. The methods comprise adding from 1 to 2500 ppm PDA and 1 to 3500ppm organic acid to the system experiencing the fouling problem.

U.S. Pat. No. 5,254,760 teaches that the polymerization of a vinylaromatic compound, such as styrene, is very effectively inhibited duringdistillation or purification by the presence of at least one stablenitroxyl compound together with at least one aromatic nitro compound.

U.S. Pat. No. 5,446,220 discloses methods for inhibiting thepolymerization of vinyl aromatic monomers in oxygen-free processingsystems. These methods comprise adding from 1 to about 10,000 parts permillion parts monomer of a combination of a dinitrophenol compound, ahydroxylamine compound and a phenylenediamine compound. Preferably,2-sec-butyl-4,6-dinitrophenol or 4,6-dinitro-o-cresol are used incombination with bis-(hydroxypropyl)hydroxylamine andN,N′-di-sec-butyl-p-phenylenediamine.

U.S. Pat. Nos. 5,545,782 and 5,545,786 disclose that nitroxyl inhibitorsin combination with some oxygen reduce the premature polymerization ofvinyl aromatic monomers during the manufacturing processes for suchmonomers. Even small quantities of air used in combination with thenitroxyl inhibitors are said to result in vastly prolonged inhibitiontimes for the monomers.

European Patent Application 0 178 168 A2 discloses a method forinhibiting the polymerization of an α,β-ethylenically unsaturatedmonocarboxylic acid during its recovery by distillation by using anitroxide free radical.

European Patent Application 0 325 059 A2 discloses stabilizing vinylaromatic compounds against polymerization by the addition of aneffective amount of a polymerization inhibition composition comprising(a) a phenothiazine compound; and (b) an aryl-substitutedphenylenediamine compound.

European Patent Application 0 398 633 A1 discloses a method ofinhibiting acid monomer polymerization comprising adding to the monomer(a) a manganese source compound and (b) a phenylenediamine compoundhaving at least one N—H bond therein.

European Patent Application 0 594 341 A1 discloses methods andcompositions for inhibiting the polymerization of vinyl aromaticmonomers under distillation conditions. The compositions comprise acombination of a phenylenediamine compound and a hydroxylamine compound.

European Patent Application 0 765 856 A1 discloses a stabilized acrylicacid composition in which the polymerization of the acrylic acid isinhibited during the distillation process for purifying or separatingthe acrylic acid as well as during transport and storage. Thecompositions comprise three components: (a) acrylic acid, (b) a stablenitroxyl radical, and (c) a dihetero-substituted benzene compound havingat least one transferable hydrogen (e.g., a quinone derivative such asthe monomethyl ether of hydroquinone (MEHQ)). During the distillationprocess, transport, and storage, components (b) and (c) are present in apolymerization-inhibiting amount. During the distillation process,oxygen (d) is preferably added with components (b) and (c). According tothe specification, examples of suitable nitroxide free radical compoundsinclude di-t-butylnitroxide; di-t-amylnitroxide;2,2,6,6-tetramethyl-piperidinyloxy;4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;4-oxo-2,2,6,6-tetramethyl-piperidinyloxy;4-dimethylamino-2,2,6,6-tetramethyl-piperidinyloxy;4-amino-2,2,6,6-tetramethyl-piperidinyloxy;4-ethanoyloxy-2,2,6,6-tetramethyl-piperidinyloxy;2,2,5,5-tetramethylpyrrolidinyloxy;3-amino-2,2,5,5-tetramethylpyrrolidinyloxy;2,2,5,5-tetramethyl-1-oxa-3-azacyclopentyl-3-oxy;2,2,5,5-tetramethyl-1-oxa-3-pyrrolinyl-1-oxy-3-carboxylic acid; and2,2,3,3,5,5,6,6-octamethyl-1,4-diazacyclohexyl-1,4-dioxy.

WO 97/46504 concerns substance mixtures containing: (A) monomerscontaining vinyl groups; and (B) an active amount of a mixture whichinhibits premature polymerization of the monomers containing vinylgroups during their purification or distillation and contains: (i)between 0.05 and 4.5 wt %, relative to the total mixture (B), of atleast one N-oxyl compound of a secondary amine which has no hydrogenatom at the α-C atoms; and (ii) between 99.95 and 95.5 wt % relative tothe total mixture (B), of at least one nitro compound. The publicationalso discloses a process for inhibiting the premature polymerization ofmonomers, and the use of mixture (B) for inhibiting the prematurepolymerization of monomers.

WO 98/02403 relates to inhibiting the polymerization of vinyl aromaticcompounds by using a mixture of a phenol and a hydroxylamine. It is saidthat the process is useful in ethylbenzene dehydrogenation effluentcondenser systems and styrene-water separator vent gas compressorsystems and that it effectively inhibits polymerization of monomers,preventing the formation of a polymer coating on condenser andcompressor equipment, thus reducing the necessity for cleaning ofequipment surfaces.

WO 98/14416 discloses that the polymerization of vinyl aromatic monomerssuch as styrene is inhibited by the addition of a composition of astable hindered nitroxyl radical and an oxime compound.

WO 98/25872 concerns substance mixtures containing: (A) compoundscontaining vinyl groups; (B) an active amount of a mixture whichinhibits premature polymerization of the compounds containing vinylgroups and contains: (i) at least one N-oxyl compound of a secondaryamine which does not carry any hydrogen atoms on the α-carbon atoms; and(ii) at least one iron compound; (C) optionally nitro compounds; and (D)optionally co-stabilizers. The publication also discloses a process forinhibiting the premature polymerization of compounds (A) containingvinyl groups, and the use of (B) optionally mixed with nitro compounds(C) and/or co-stabilizers (D) for inhibiting the prematurepolymerization of radically polymerizable compounds and stabilizingorganic materials against the harmful effect of radicals.

U.K. Patent Number 1,127,127 discloses that acrylic acid can bestabilized against polymerization by the addition thereto of a nitroxidehaving the essential skeletal structure:

wherein R₁, R₂, R₃, and R₄ are alkyl groups and no hydrogen is bound tothe remaining valencies on the carbon atoms bound to the nitrogen. Thetwo remaining valencies that are not satisfied by R₁ to R₄ or nitrogencan also form part of a ring (e.g., 2,2,6,6tetramethyl-4-hydroxy-piperidine-1-oxyl).

CS-260755 B1 is directed to the preparation of4-substituted-2,2,6,6-tetramethylpiperidine nitroxyls as olefinstabilizers.

SU-334845 A1 is directed to the inhibition of the radical polymerizationof oligoester acryl-ates using iminoxyl radical inhibitors of a givenformula.

SU-478838 is directed to the inhibition of the radical polymerization ofoligoester acrylates and the prevention of oligomeric peroxides using abinary polymerization inhibitor comprising quinone.

FR 2,761,060 relates to the prevention of premature polymerization ofstyrene during its production by dehydrogenation of ethylbenzene byinjecting into the process effluent a radical inhibitor based on anoxyl-tetramethylpiperidine derivative.

The foregoing are incorporated herein by reference in their entirety.

SUMMARY OF THE INVENTION

In accordance with the present invention, inhibiting systems have beendeveloped in which a component that is a hydrogen donor or electronacceptor or a combination of two or more of such components is used inthe purification train, either alone or, preferably, in combination witha nitroxyl radical to prevent polymer growth via a “living”polymerization mechanism. When the component is used in combination withthe nitroxyl radical, the effectiveness of the nitroxyl radicalinhibitor can be preserved and utilized without risking high molecularweight polymer formation and/or coating of the internal parts of thepurification train owing to excessive polymer growth over time.

More particularly, the present invention is directed to a method forinhibiting the premature polymerization and the polymer growth ofethylenically unsaturated monomers comprising adding to said monomers aneffective amount of at least one inhibitor that is a hydrogen donor oran electron acceptor.

It is also advantageous to add a transition metal ion to the monomers.The preferred transition metal ion is copper, especiallyCu(I)naphthenate.

In a preferred embodiment, the present invention is directed to a methodfor inhibiting the premature polymerization and the polymer growth ofethylenically unsaturated monomers comprising adding to said monomers A)an effective amount of at least one first inhibitor that is a hydrogendonor or an electron acceptor and B) at least one second inhibitorhaving the following structural formula:

In another aspect, the present invention is directed to a compositioncomprising A) at least one first inhibitor that is a hydrogen donor oran electron acceptor and B) at least one second inhibitor having thefollowing structural formula:

In formula (I), R₁ and R₄ are independently selected from the groupconsisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R₂and R₃ are independently selected from the group consisting of alkyl andheteroatom-substituted alkyl; and X₁ and X₂ (1) are independentlyselected from the group consisting of halogen, cyano, COOR₇, —S—COR₇,—OCOR₇, (wherein R₇ is alkyl or aryl), amido, —S—C₆H₅, carbonyl,alkenyl, or alkyl of 1 to 15 carbon atoms, or (2) taken together, form aring structure with the nitrogen, preferably of five, six, or sevenmembers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As stated above, the present invention is directed to inhibiting systemsin which at least one hydrogen donor or electron acceptor is used in thepurification train, preferably in addition to at least one nitroxylradical, to prevent polymer growth that occurs via a “living”polymerization mechanism.

The hydrogen donor compounds can, for example, be hydroxylamines,oximes, phenols, catechols, hydroquinones, thiols, anilines,dihydroanthracenes, and the like. Such compounds can include a metalspecies which facilitates the reduction/oxidation reactions that canaccompany growth inhibition through deactivation of the growing radicalchain. More particularly, the hydrogen donor compounds are preferablychosen from compounds having the structural formulae I through V.

In structural formulae I through V:

-   R₁₀₀ and R₁₀₁ are independently selected from the group consisting    of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR₁₀₂,    COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substituted alkyl or    aryl where the substituents are C, O, N, S, or P, or    -   R₁₀₀ and R₁₀₁ can be taken together to form a ring structure of        five to seven members;-   R₁₀₂ and R₁₀₃ are independently selected from the group consisting    of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and    substituted alkyl or aryl where the substituents are C, O, N, S, or    P, or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure    of five to seven members;-   R₁₀₄, R₁₀₅, R₁₀₆, R₁₀₇, R₁₀₈, and R₁₀₉ are independently selected    from the group consisting of hydrogen, alkyl, aryl, cycloalkyl,    heterocyclic, substituted alkyl, substituted aryl, OR₁₁₀, NR₁₁₀R₁₁₁,    SR₁₁₀, NO₂, NO, CN, COR₁₁₂, halogen (as used herein, halogen    includes fluorine, chlorine, bromine, and iodine), and/or any two    adjacent groups can be taken together to form ring structure(s) of    five to seven members, provided that at least one of R₁₀₄, R₁₀₅,    R₁₀₆, R₁₀₇, R₁₀₈, and R₁₀₉ is OH or NHR₁₁₀;-   R₁₁₀ and R₁₁₁ are independently selected from the group consisting    of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, substituted    alkyl or aryl where the substituents are C, O, N, S, or P, and    COR₁₀₂, or R₁₁₀ and R₁₁₁ can be taken together to form a ring    structure of five to seven members;-   R₁₁₂ is R₁₀₂, OR₁₀₂, or NR₁₀₂R₁₀₃;-   R₁₁₃, R₁₁₄, and R₁₁₅ are independently selected from the group    consisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclic    moieties; and-   R₁₁₆, R₁₁₇, R₁₁₈, R₁₁₉, R₁₂₀, R₁₂₁, R₁₂₂, and R₁₂₃ are independently    selected from the group consisting of hydrogen, alkyl, aryl,    cycloalkyl, heterocyclic, substituted alkyl, substituted aryl,    OR₁₁₀, NR₁₁₀R₁₁₁, SR₁₁₀, NO₂, NO, CN, COR₁₁₂, halogen, and/or any    two adjacent groups can be taken together to form ring structure(s)    of five to seven members.

The electron accepting compounds can, for example, be quinones, quinoneimines, quinone methides, and acetylenes. Such compounds can include ametal species which facilitates the reduction/oxidation reactions thatcan accompany growth inhibition through deactivation of the growingradical chain. More particularly, the electron accepting compounds arepreferably chosen from compounds having the structural formulae VI orVII.

In structural formula VI:

-   X and Y are independently selected from the group consisting of    oxygen, NR₁₁₀, and CR₁₂₄R₁₂₅;-   R₁₂₀, R₁₂₁, R₁₂₂, and R₁₂₃ are independently selected from the group    consisting of hydrogen, alkyl, aryl, cycloalkyl, heterocyclic,    substituted alkyl, substituted aryl, OR₁₁₀, NR₁₁₀R₁₁₁, SR₁₁₀, NO,    NO₂, CN, COR₁₁₂, and halogen, or R₁₂₀ and R₁₂₁, can be taken    together and/or R₁₂₂ and R₁₂₃ can be taken together to form one or    two ring structures, respectively, either of which can be of five to    seven members;-   R₁₂₄ and R₁₂₅ are independently selected from the group consisting    of hydrogen, alkyl, aryl, cycloalkyl, heterocyclic, substituted    alkyl, substituted aryl, OR₁₁₀, NR₁₁₀R₁₁₁, SR₁₁₀, NO₂, NO, CN,    COR₁₁₂, halogen, and/or can be taken together to form a ring    structure of five to seven members;-   R₁₁₀ and R₁₁₁ are independently selected from the group consisting    of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, substituted    alkyl or aryl where the substituents are C, O, N, S, or P, and    COR₁₀₂, or R₁₁₀ and R₁₁₁ can be taken together to form a ring    structure of five to seven members;-   R₁₁₂ is R₁₀₂, OR₁₀₂, or NR₁₀₂R₁₀₃; and-   R₁₀₂ and R₁₀₃ are independently selected from the group consisting    of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and    substituted alkyl or aryl where the substituents are C, O, N, S, or    P, or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure    of five to seven members.

In structural formula VII:

-   R₁₂₆ and R₁₂₇ are independently selected from the group consisting    of hydrogen, alkyl, aryl, cycloalkyl, heterocyclic, substituted    alkyl, substituted aryl, OR₁₁₀, NR₁₁₀R₁₁₁, SR₁₁₀, NO₂, NO, CN,    COR₁₁₂, and halogen wherein R₁₁₀, R₁₁₁, and R₁₁₂ are defined as for    formula VI.

In the foregoing, alkyl (or substituted alkyl) groups preferably contain1 to 15 carbon atoms, e.g., methyl, ethyl, propyl, butyl, pentyl, hexyl,heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl,pentadecyl, and the like, and isomers thereof, e.g., t-butyl,2-ethylhexyl, and the like. It is more preferred that the alkyl (orsubstituted alkyl) groups be of one to five carbon atoms (e.g., methyl,ethyl, propyl, butyl, pentyl, and isomers thereof). Substituents on thesubstituted alkyl groups can be any moiety that will not interfere withthe hydrogen donating or electron receiving functions of the compounds.Aryl groups are preferably of from 6 to 10 carbon atoms, e.g., phenyl ornaphthyl, which, in addition, may be substituted with non-interferingsubstituents, e.g., lower alkyl groups, halogens, and the like.

Exemplary hydrogen donating compounds include, but are not limited to,diethylhydroxylamine, cyclohexanoneoxime, dibenzylhydroxylamine,2,4-dinitro-6-sec-butylphenol,N-phenyl-N′-(1,4-dimethylpentyl)-para-phenylenediamine,2,5-di-t-butylhydroquinone, 2,5-di-t-amylhydroquinone,methylhydroquinone, 4-t-butylhydroquinone, 4-t-butylcatechol,octanethiol, 2,6-di-t-butyl-4-ethylphenol/Cu(I)naphthenate,dihydroanthracene, N-t-butyl-2-benzothiazole-sulfenamide,N-methyl-4-nitroaniline, and the like.

Exemplary electron accepting compounds include, but are not limited to,phenylacetylene, 2,5-di-t-butyl-1,4-benzoquinone,2,6-di-t-butyl-1,4-benzoquinone, 1,4-benzoquinone,2-methylanthraquinone, 1,4-naphthoquinone,2,6-di-t-butyl-4-(phenylmethylene)-2,5-cyclohexadiene-1-one,2,6-di-t-butyl-4-(phenylimino)-2,5-cyclohexadiene-1-one, ethyl3,4-bis-(3,5-di-t-butyl-4-one-2,5-cyclohexadienylidene)-hexane-1,6-dioate,and the like.

An effective growth inhibiting system can consist of one or more of anyof the compounds described above with or without one or more nitroxylcompounds.

As stated above, in one preferred aspect, the present invention isdirected to a method for inhibiting the premature polymerization ofethylenically unsaturated monomers comprising adding to said monomers,in addition to at least one first inhibitor that is a hydrogen donor oran electron acceptor, an effective amount of at least one secondinhibitor that is a stable hindered nitroxyl compound having thestructural formula:

wherein R₁ and R₄ are independently selected from the group consistingof hydrogen, alkyl, and heteroatom-substituted alkyl and R₂ and R₃ areindependently selected from the group consisting of alkyl andheteroatom-substituted alkyl; and X, and X₂ (1) are independentlyselected from the group consisting of halogen, cyano, COOR₇, —S—COR₇,—OCOR₇, (wherein R₇ is alkyl or aryl), amido, —S—C₆H₅, carbonyl,alkenyl, or alkyl of 1 to 15 carbon atoms, or (2) taken together, form aring structure with the nitrogen.

In a particularly preferred embodiment, the stable hindered nitroxylcompound has the structural formula:

wherein R₁ and R₄ are independently selected from the group consistingof hydrogen, alkyl, and heteroatom-substituted alkyl and R₂ and R₃ areindependently selected from the group consisting of alkyl andheteroatom-substituted alkyl, and the

portion represents the atoms necessary to form a five-, six-, orseven-membered heterocyclic ring.

Accordingly, one of the several classes of cyclic nitroxides that can beemployed in the practice of the present invention can be represented bythe following structural formula:

wherein Z₁, Z₂, and Z₃ are independently selected from the groupconsisting of oxygen, sulfur, secondary amines, tertiary amines,phosphorus of various oxidation states, and substituted or unsubstitutedcarbon atoms, such as >CH₂, >CHCH₃, >C═O,>C(CH₃)₂, >CHBr, >CHCl, >CHI, >CHF, >CHOH, >CHCN, >C(OH)CN, >CHCOOH,>CHCOOCH₃, >CHCOOC₂H₅, >C(OH)COOC₂H₅, >C(OH)COOCH₃, >C(OH)CHOHC₂H₅,>CR₅OR₆, >CHNR₅R₆, >CCONR₅R₆, >C═NOH, >C═CH—C₆H₅, >CF₂, >CCl₂, >CBr₂,>CI₂, >CR₅PR₁₃R₁₄R₁₅, and the like, where R₅ and R₆ are independentlyselected from the group consisting of hydrogen, alkyl, aryl, and acyland R₁₃, R₁₄, and R₁₅ are independently selected from the groupconsisting of unshared electrons, alkyl, aryl, ═O, OR₁₆, and NR₁₇R₁₈,where R₁₆, R₁₇, and R₁₈ are independently selected from the groupconsisting of hydrogen, alkyl, and aryl. Where R₅ and/or R₆ are alkyl,it is preferred that they be a lower alkyl (i.e., one having one to fivecarbon atoms, e.g., methyl, ethyl, propyl, butyl, pentyl, and isomersthereof).

Where R₅ and/or R₆ are aryl, it is preferred that they be aryl of from 6to 10 carbon atoms, e.g., phenyl or naphthyl, which, in addition, may besubstituted with non-interfering substituents, e.g., lower alkyl groups,halogens, and the like.

Where R₅ and/or R₆ are acyl, it is preferred that they be acyl of thestructure

where R₁₉ is alkyl, aryl, OR₂₀, or NR₂₀R₂₁ and where R₂₀ and R₂₁ arealkyl, aryl, or

where R₂₂ is alkyl or aryl. Where R₁₉, R₂₀, R₂₁, or R₂₂ are alkyl, theyare preferably alkyl of from 1 to 15 carbon atoms, more preferably loweralkyl of from 1 to 5 carbon atoms, as described above. Where R₁₉, R₂₀,R₂₁, or R₂₂ are aryl, they are preferably aryl of from 6 to 10 carbonatoms, as described above.

Another of the several classes of cyclic nitroxides that can be employedin the practice of the present invention can be represented by thefollowing structural formula:

wherein Z₁ and Z₂, which may be the same or different, are nitrogen orsubstituted or unsubstituted carbon atoms, such as ═C(H)—, ═C(CH₃)—,═C(COOH)—, ═C(COOCH₃)—, ═C(COOC₂H₅)—, ═C(OH)—, ═C(CN)—, ═C(NR₅R₆)—,═C(CONR₅R₆)—, and the like, and where Z₃, R₅, and R₆ are as describedabove.

The cyclic nitroxides employed in the practice of the present inventioncan also be derived from five-membered rings. These compounds are of thestructure:

wherein Z₂ and Z₃, which may be the same or different, are sulfur,oxygen, secondary amines, tertiary amines, phosphorus of variousoxidation states, or substituted or unsubstituted carbon atoms, such as,>CH₂, >CHCH₃, >C═O,>C(CH₃)₂, >CHBr, >CHCl, >CHI, >CHF, >CHOH, >CHCN, >C(OH)CN, >CHCOOH,>CHCOOCH₃, >CHCOOC₂H₅, >C(OH)COOC₂H₅, >C(OH)COOCH₃, >C(OH)CHOHC₂H₅,>CROR₆, >CHNR₅R₆, >CCONR₅R₆, >C═NOH, >C═CH—C₆H₅, CF₂, CCl₂, CBr₂, CI₂,>CR₅PR₁₃R₁₄R₁₅, and the like, wherein the several R groups are asdescribed above.

The cyclic nitroxides employed in the practice of the present inventioncan also have the structure:

wherein Z₄ and Z₅, which can be the same or different, can be nitrogenor a substituted or unsubstituted carbon atom, such as ═C(H)—, ═C(CH₃)—,═C(COOH)—, ═C(COOCH₃)—, ═C(COOC₂H₅)—, ═C(OH)—, ═C(CN)—, ═C(NR₅R₆)—,═C(CONR₅R₆)—, and the like, where R₅ and R₆ are as described above.

Another class of cyclic nitroxides that can be employed in the practiceof the present invention is of the structure:

wherein Z₂ and Z₃, which may be the same or different, are sulfur,oxygen, secondary amines, tertiary amines, or substituted orunsubstituted carbon atoms, such as, >CH₂, >CHCH₃, >C═O,>C(CH₃)₂, >CHBr, >CHCl, >CHI, >CHF, >CHOH, >CHCN, >C(OH)CN, >CHCOOH,>CHCOOCH₃, >CHCOOC₂H₅, >C(OH)COOC₂H₅, >C(OH)COOCH₃, >C(OH)CHOHC₂H₅,>CHNR₅R, >CCONR₅R₆, >CR₅OR₆, >C═NOH, >C═CH—C₆H₅, CF₂, CCl₂, CBr₂, CI₂,>CR₅PR₁₃R₁₄R₁₅, and the like, where the several R groups are asdescribed above.

Further, two or more nitroxyl groups can be present in the samemolecule, for example, by being linked through one or more of the Z-typemoieties by a linking group E, as disclosed in U.S. Pat. No. 5,254,760,which is incorporated herein by reference.

As stated above, for all the nitroxyl structures above, R₁ and R₄ areindependently selected from the group consisting of hydrogen, alkyl, andheteroatom-substituted alkyl and R₂ and R₃ are independently selectedfrom the group consisting of alkyl and heteroatom-substituted alkyl. Thealkyl (or heteroatom-substituted alkyl) groups R₁ through R₄ can be thesame or different and preferably contain 1 to 15 carbon atoms, e.g.,methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl,decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, and the like,and isomers thereof, e.g., t-butyl, 2-ethylhexyl, and the like. It ismore preferred that R₁ through R₄ be independently selected lower alkyl(or heteroatom-substituted lower alkyl) of one to five carbon atoms(e.g., methyl, ethyl, propyl, butyl, pentyl, and isomers thereof). Whereheteroatom substituents are present, they can, for example, includehalogen, oxygen, sulfur, nitrogen, and the like. It is most preferredthat all of R₁ through R₄ be methyl.

Examples of suitable nitroxide free radical compounds that can be usedin combination with the hydrogen donor or electron acceptor in thepractice of the present invention, include, but are not limited to:

-   N,N-di-tert-butylnitroxide;-   N,N-di-tert-amylnitroxide;-   N-tert-butyl-2-methyl-1-phenyl-propylnitroxide;-   N-tert-butyl-1-diethylphosphono-2,2-dimethylpropylnitroxide;-   2,2,6,6-tetramethyl-piperidinyloxy;-   4-amino-2,2,6,6-tetramethyl-piperidinyloxy;-   4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;-   4-oxo-2,2,6,6-tetramethyl-piperidinyloxy;-   4-dimethylamino-2,2,6,6-tetramethyl-piperidinyloxy;-   4-ethanoyloxy-2,2,6,6-tetramethyl-piperidinyloxy;-   2,2,5,5-tetramethylpyrrolidinyloxy;-   3-amino-2,2,5,5-tetramethylpyrrolidinyloxy;-   2,2,4,4-tetramethyl-1-oxa-3-azacyclopentyl-3-oxy;-   2,2,4,4-tetramethyl-1-oxa-3-pyrrolinyl-1-oxy-3-carboxylic acid;-   2,2,3,3,5,5,6,6-octamethyl-1,4-diazacyclohexyl-1,4-dioxy;-   4-bromo-2,2,6,6-tetramethyl-piperidinyloxy;-   4-chloro-2,2,6,6-tetramethyl-piperidinyloxy;-   4-iodo-2,2,6,6-tetramethyl-piperidinyloxy;-   4-fluoro-2,2,6,6-tetramethyl-piperidinyloxy;-   4-cyano-2,2,6,6-tetramethyl-piperidinyloxy;-   4-carboxy-2,2,6,6-tetramethyl-piperidinyloxy;-   4-carbomethoxy-2,2,6,6-tetramethyl-piperidinyloxy;-   4-carbethoxy-2,2,6,6-tetramethyl-piperidinyloxy;-   4-cyano-4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;-   4-methyl-2,2,6,6-tetramethyl-piperidinyloxy;-   4-carbethoxy-4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;-   4-hydroxy-4-(1-hydroxypropyl)-2,2,6,6-tetramethyl-piperidinyloxy;-   4-methyl-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;-   4-carboxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;-   4-carbomethoxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;-   4-carbethoxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;-   4-amino-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;-   4-amido-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;-   3,4-diketo-2,2,5,5-tetramethylpyrrolidinyloxy;-   3-keto-4-oximino-2,2,5,5-tetramethylpyrrolidinyloxy;-   3-keto-4-benzylidine-2,2,5,5-tetramethylpyrrolidinyloxy;-   3-keto-4,4-dibromo-2,2,5,5-tetramethylpyrrolidinyloxy;-   2,2,3,3,5,5-hexamethylpyrrolidinyloxy;-   3-carboximido-2,2,5,5-tetramethylpyrrolidinyloxy;-   3-oximino-2,2,5,5-tetramethylpyrrolidinyloxy;-   3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;-   3-cyano-3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;-   3-carbomethoxy-3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;-   3-carbethoxy-3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;-   2,2,5,5-tetramethyl-3-carboxamido-2,5-dihydropyrrole-1-oxyl;-   2,2,5,5-tetramethyl-3-amino-2,5-dihydropyrrole-1-oxyl;-   2,2,5,5-tetramethyl-3-carbethoxy-2,5-dihydropyrrole-1-oxyl;-   2,2,5,5-tetramethyl-3-cyano-2,5-dihydropyrrole-1-oxyl;-   bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)succinate;-   bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipate;-   bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)sebacate;-   bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)n-butylmalonate;-   bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)phthalate;-   bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)isophthalate;-   bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)terephthalate;-   bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)hexahydroterephthalate;-   N,N′-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipamide;-   N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-caprolactam;-   N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-dodecylsuccinimide;-   2,4,6-tris-[N-butyl-N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)]-s-triazine;-   4,4′-ethylenebis(1-oxyl-2,2,6,6-tetramethylpiperazin-3-one); and the    like.

As used herein, the abbreviation TEMPO stands for2,2,6,6-tetramethyl-1-piperidinyloxy. Thus, 4-amino-TEMPO is4-amino-2,2,6,6-tetramethyl-1-piperidinyloxy; 4-hydroxy-TEMPO is4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (also known in the art asHTEMPO); 4-oxo-TEMPO is 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy; andso on.

It is preferred that one member of the combination employed in thepractice of the present invention be 4-amino-TEMPO, 4-oxo-TEMPO,4-hydroxy-TEMPO, or TEMPO.

Blends of two or more of the foregoing, e.g., 4-amino-TEMPO and4-oxo-TEMPO, can also be employed.

Such stable nitroxide free radical compounds can be prepared by knownmethods. (See, for example, U.S. Pat. Nos. 3,163,677; 3,334,103;3,372,182; 3,422,144; 3,494,930; 3,502,692; 3,873,564; 3,966,711; and4,665,185; which are incorporated herein by reference.) They aresuitable for use over a wide range of temperatures, but distillationtemperatures employed with the ethylenically unsaturated monomers thatare stabilized by the process of the present invention typically rangefrom about 60° C. to about 180° C., preferably from about 70° C. toabout 165° C., and, more preferably, from about 80° C. to about 150° C.Such distillations are generally performed at an absolute pressure inthe range of about 10 to about 1,200 mm of Hg.

The ethylenically unsaturated monomer, the premature polymerization andpolymer growth of which is an object of the present invention, can beany such monomer for which unintended polymerization and/or polymergrowth during its manufacture, storage, and/or distribution is aproblem. Among those monomers that will benefit from the practice of thepresent invention are: styrene, α-methylstyrene, styrene sulfonic acid,vinyltoluene, divinylbenzenes, polyvinylbenzenes, alkylated styrene,2-vinylpyridine, acrylonitrile, methacrylonitrile, methyl acrylate,ethyl acrylate, methyl methacrylate, ethyl methacrylate, acrylic acid,methacrylic acid, butadiene, chloroprene, isoprene, and the like.

The ethylenically unsaturated monomers will not necessarily bestabilized indefinitely by the presence of the inhibitor(s), especiallywhen the monomers are heated as in distillation, but they can beconsidered to be stabilized as long as A) there is a measurable increasein the time for which they can be heated before the onset ofpolymerization and/or polymer growth in a static system, B) the amountof polymer made at a constant temperature remains constant over time ina dynamic system, and/or C) the rate of polymer growth is significantlyslower than when the growth inhibiting system is not present.

Those skilled in the art will understand that, if desired, free radicalscavengers can also be included in the practice of the presentinvention. For example, air or O₂, as disclosed in U.S. Pat. Nos.5,545,782 and 5,545,786, can be added, as can the aromatic nitrocompounds disclosed in U.S. Pat. No. 5,254,760, the dihetero-substitutedbenzene compounds having at least one transferable hydrogen, e.g., aquinone derivative such as the mono-methyl-ether of hydroquinonedisclosed in European Patent Application 0 765 856 A1, the ironcompounds disclosed in WO 98/25872, and other inhibitors, e.g.,phenolics and certain inorganic salts, well-known to those skilled inthe art.

The polymerization inhibitor(s) can be introduced into the monomer to beprotected by any conventional method. They can, for example, be added asa concentrated solution in suitable solvents just upstream from thepoint of desired application by any suitable means. In addition,individual inhibiting components can be injected separately into thedistillation train along with the incoming feed and/or through separateand multiple entry points, provided there is an efficient distributionof the inhibiting composition. Since the inhibitors are graduallydepleted during the distillation operation, it is generally advantageousto maintain the appropriate amount of them in the distillation apparatusby adding them during the course of the distillation process. Addinginhibitors can be done either on a generally continuous basis orintermittently, in order to maintain the inhibitor concentration abovethe minimum required level.

The total inhibitor concentration should be from about 1 to about 2000ppm versus the monomer being inhibited; preferably from about 5 to about1000 ppm, depending on the conditions of use.

The ratio of the first component, or blend A (electron acceptor orhydrogen donor compound or blend thereof) to a second component, orblend B (nitroxyl or nitroxyls), based on the total of both componentsis from about 1 to 100 wt % A: about 99 to 0 wt % B; preferably, about25-75 wt % A: about 75-25 wt % B; more preferably about 50-75 wt % A:about 50-25 wt % B.

The advantages and the important features of the present invention willbe more apparent from the following examples.

EXAMPLES Procedure for Polymer Growth Reboiler Test Preparation of FeedSolution

Tert-butylcatechol (TBC) is removed from commercially available styreneby distillation under vacuum. Removal of TBC is verified by caustictitration. The desired amount of inhibitor(s) is added to this TBC-freestyrene either directly or by first making a concentrated solution ofthe inhibitor in TBC-free styrene followed by further dilution withTBC-free styrene.

Procedure for Polymer Growth Dynamic Reboiler Test

A quantity of the Feed Solution containing inhibitor or blend ofinhibitors at the desired charge (stated as a wt/wt total inhibitor tostyrene) is added to a round-bottom flask (the Pot). A known quantity ofinsoluble polymer capable of growing via a living mechanism is placedinside the Pot and submersed in the Feed Solution in the Pot. Theinsoluble polymer can be retained in the Pot by any suitable means.Typically, the insoluble polymer is securely wrapped in a piece offilter paper or wire mesh and suspended by a wire within the Pot.Conversely, the Bottoms Stream line (as described below) can be coveredwith filter paper or mesh to prevent insoluble polymer from beingremoved from the Pot. The Pot is placed in a hot oil bath, and the FeedSolution in the pot is heated to the desired temperature (usually 130°C) and brought to reflux by adjusting the pressure/vacuum. Once the Potcontents are at temperature, a continuous stream of fresh Feed Solutionis begun at a rate that will add the volume of the initial Pot solutionto the Pot over a period of time called the “residence time” (typicallyone hour). At the same time at which the fresh Feed Solution flow isbegun, the Bottoms Stream flow is also begun. The Bottoms Stream issolution in the Pot that is removed at the same rate as the fresh FeedSolution is added. The equal flows of Feed and Bottoms Streams causesthe quantity in the Pot to remain constant over the time of theexperiment while allowing continuous replenishment of inhibitor. Thisprocedure simulates the way inhibitors are used in a distillation trainof a plant producing vinyl monomers. The experiment continues with flowin and out of the Pot for a specified length of time (usually 7 hours).Samples are collected hourly from the Bottoms Stream. These samples areanalyzed for polymer content via the methanol turbidity method. Theamount of polymer in the samples is an indication of effectiveness ofthe inhibitor system being tested.

After running for the specified length of time, the vacuum is releasedand, if used, the filter paper bag of polymer is removed. The Potsolution is filtered to recover any insoluble polymer that may haveescaped from the bag. Any filtered polymer and the polymer in the filterpaper bag are allowed to dry open to the atmosphere for at least 18hours. The polymer can be further dried by placing it in a vacuum ovenat 40-50° C. under full vacuum for 1-2 hours. The polymer is thenweighed. Percent growth is determined by the following equation:% growth=weight of final insoluble polymer−weight of initial insolublepolymer×100 weight of initial insoluble polymer

Lower percent growth numbers indicate increased effectiveness of thesystem to inhibit polymer growth via a “living” mechanism.

Preparation of Insoluble Polymer Capable of Growing

Tert-butylcatechol (TBC) was removed from commercially available styreneand from commercially available divinylbenzene (DVB) by distillationunder vacuum. Removal of TBC was verified by caustic titration. TBC-freestyrene (50 g), ethylbenzene (49 g), TBC-free DVB (1 g), and 4-oxo-TEMPO(0.01 g) were combined. The mixture was stirred at 130° C. until themixture polymerized to a gel (about 3 hours). The gel-like system wascooled to about 60° C., and 2 liters of ethylbenzene were added. Theresulting mixture was stirred for 2 hours at 50° C., filtered by vacuumfiltration until the gel was mostly dry, and remaining solvent wasremoved by evaporation under full vacuum at 50° C. A hard, white polymerwas obtained (25 g, 49% yield).

Procedure for Multi-Pass Reboiler Test

Preparation of First Pass Feed Solution:

Tert-butylcatechol (TBC) is removed from commercially available styreneby distillation under vacuum. Removal of TBC is verified by caustictitration. The desired amount of inhibitor(s) is added to this TBC-freestyrene either directly or by first making a concentrated solution ofthe inhibitor in TBC-free styrene followed by further dilution withTBC-free styrene.

Procedure for Reboiler Test (a Dynamic Test):

A quantity of the Feed Solution containing inhibitor (blend) at thedesired charge (stated as a wt/wt total inhibitor to styrene) is addedto a round-bottom flask (the Pot) and heated to the desired temperature(usually 130° C.) and brought to reflux by adjusting thepressure/vacuum. Once the Pot contents are at temperature, a continuousstream of fresh Feed Solution is begun at a rate that will add thevolume of the initial Pot solution to the Pot over a period of timecalled the residence time (typically, one hour). At the same time thatthe fresh Feed Solution flow is begun, the Bottoms Stream flow is alsobegun. The Bottoms Stream is solution in the Pot that is removed at thesame rate as the fresh Feed Solution is added. The equal flows of Feedand Bottoms Streams cause the quantity in the Pot to remain constantover the time of the experiment, while allowing continuous replenishmentof inhibitor. This procedure simulates the way inhibitors are used in adistillation train of a plant producing vinyl monomers. The experimentcontinues with flow in and out of the Pot for a specified period oftime. Typically, the First Pass runs for 10 hours, the Second Pass runsfor 9 hours, the Third Pass runs for 8 hours, etc.

Samples are collected hourly from the Bottoms Stream. These samples areanalyzed for polymer content via the methanol turbidity method. Theamount of polymer in the samples is an indication of effectiveness ofthe inhibitor system being tested. “Average Polymer Make” is the averageof the polymer content values for samples taken after 4 hours running.

The material left in the Pot at the end of the run is quickly removedand cooled, to stop any further polymerization. The material is thenconcentrated, if necessary, under reduced pressure at 40° C. until thepolymer content is >5 wt %. A sample of this polymer solution is thenanalyzed by Gel Permeation Chromatography (GPC) to determine theweighted average molecular weight (M_(w)) of the polymer.

Preparation of Second and Third Pass Feed Solutions:

The Bottoms Stream from the previous Pass is collected except for thematerial in the Pot at the end of the run. The amounts of inhibitor(s)in the First Pass Feed Solution and the Bottoms Stream from the FirstPass are determined by appropriate analytical method(s), e.g., gaschromatography. An amount of inhibitor(s) is added to the collectiveBottoms Stream from the First Pass to increase the level of inhibitor(s)in the Bottoms Stream to a level equal to that found in the First PassFeed Solution. An equivalent amount of inhibitor(s) is added to thecollective Bottoms Streams for subsequent Passes.

Evaluation of the Results

The difference in the “Average Polymer Make” made in one Pass versussubsequent Passes is an indication of the ability of the inhibitingsystem to prevent or allow polymer to grow. For example, an increase inthe amount of polymer made going from one Pass to the next which isroughly equivalent to the amount of polymer made during the First Passis an indication that the inhibiting system effectively prevents polymergrowth during recycle. Conversely, an increase in the amount of polymermade going from one Pass to the next that is dramatically greater (about10 times or more) than the amount of polymer made during the First Passis an indication that the inhibiting system does not effectively preventpolymer growth during recycle.

The difference in the M_(w) of the polymer made in one Pass versussubsequent Passes is an indication of the ability of the inhibitingsystem to prevent or allow polymer to grow. Any significant increase inM_(w) of the polymer made in one Pass versus the previous Pass is anindication that the inhibiting system does not prevent polymer growth.The closer to zero the increase in M_(w), the better the growthinhibiting ability of the system.

The effectiveness of hydrogen donor systems and their blends withnitroxyls is shown in Tables 1 and 4. The effectiveness ofelectron-accepting systems and their blends with nitroxyls is shown inTable 2. Examples of Synergistic Blends of Donor and Acceptor systemsare shown in Table 3. The first two examples in each of Tables 1-3 arebaseline examples of nitroxyl inhibitors used alone—conditions that areknown to allow polymer growth via a “living” mechanism. Under thesePolymer Growth Test baseline conditions, about 700% growth was observed.All other examples in Tables 1-3 gave less than 700% growth, indicatingthat those systems provided some growth inhibiting activity.

The first example in Table 4 is also a baseline example of a nitroxylinhibitor used alone—conditions that are known to allow polymer growthvia a “living” mechanism. Under these Multi-Pass Test baselineconditions, the average polymer make increased 100-fold and themolecular weight (M_(w)) of the polymer made increased nearly 10-foldover three passes. The other examples in Table 4 gave minor increases inaverage polymer make (versus the baseline example) and essentially nochange in molecular weight of the polymer over three passes, indicatingthat those systems provided significant growth inhibiting activity.TABLE 1 Performance of Hydrogen Donor Systems Using Polymer Growth TestMethod Growth (% increase in Inhibitor Charge(s) weight of insolubleInhibitor System (ppm vs styrene) polymer after 7 hrs.) 4-oxo-TEMPO(baseline)  300 684 4-hydroxy-TEMPO (baseline)  300 7364-oxo-TEMPO/diethylhydroxylamine 300/3000 204-oxo-TEMPO/diethylhydroxylamine 300/600 764-oxo-TEMPO/cyclohexanoneoxime 300/3000 3824-oxo-TEMPO/dibenzylhydroxylamine 300/600 388 4-oxo-TEMPO/DNBP 150/1500−2 DNBP 1500 20 DNBP/PDA 900/600 11 DNBP/PDA (air) 900/600 (8 cc/min)−13 4-oxo-TEMPO/2,5-di-t-butylhydroquinone 300/3000 1754-oxo-TEMPO/2,5-di-t-butylhydroquinone 300/600 1974-oxo-TEMPO/2,5-di-t-amylhydroquinone 300/900 1734-oxo-TEMPO/2,5-di-t-amylhydroquinone 300/600 2754-oxo-TEMPO/methylhydroquinone 300/600 4204-oxo-TEMPO/4-t-butylhydroquinone 300/300 4644-oxo-TEMPO/4-t-butylcatechol, 300/3000 56 4-oxo-TEMPO/octanethiol300/3000 220 4-oxo-TEMPO/octanethiol 300/1500 4204-oxo-TEMPO/2,6-di-t-butyl-4-methylphenol/Cu(I)naphthenate 300/3000/150416 4-oxo-TEMPO/dihydroanthracene 300/3000 5244-oxo-TEMPO/N-t-butyl-2-benzothiazole-sulfenamide 300/3000 5324-oxo-TEMPO/N-methyl-4-nitroaniline 300/3000 538PDA = N-phenyl-N′-(1,4-dimethylpentyl)-para-phenylenediamineDNBP = 2,4-dinitro-6-sec-butylphenol

TABLE 2 Performance of Electron Acceptor Systems Using Polymer GrowthTest Method Growth (% increase in Inhibitor Charge(s) weight ofinsoluble Inhibitor System (ppm vs styrene) polymer after 7 hrs.)4-oxo-TEMPO (baseline)  300 684 4-hydroxy-TEMPO (baseline)  300 7364-oxo-TEMPO/phenylacetylene 300/3000 5404-oxo-TEMPO/2,5-di-t-butyl-1,4-benzoquinone 300/3000 964-oxo-TEMPO/2,5-di-t-butyl-1,4-benzoquinone 300/600 1804-oxo-TEMPO/2,6-di-t-butyl-1,4-benzoquinone 150/1500 3584-oxo-TEMPO/1,4-benzoquinone 300/600 1364-oxo-TEMPO/2-methylanthraquinone 300/600 2354-oxo-TEMPO/1,4-naphthoquinone 300/600 3084-oxo-TEMPO/2,6-di-t-butyl-4-(phenylmethylene)-2,5- 150/1500 14cyclohexadiene-1-one2,6-di-t-butyl-4-(phenylmethylene)-2,5-cyclohexadiene-1-one 1500 404-oxo-TEMPO/2,6-di-t-butyl-4-(phenylimino)-2,5- 300/2900 396cyclohexadiene-1-one 4-oxo-TEMPO/ethyl3,4-bis-(3,5-di-t-butyl-4-one-2,5- 300/600 525cyclohexadienylidene)-hexane-1,6-dioate

TABLE 3 Performance of Synergistic Blends of Donors and Acceptors UsingPolymer Growth Test Method Growth (% increase in Inhibitor Charge(s)weight of insoluble Inhibitor System (ppm vs styrene) polymer after 7hrs.) 4-oxo-TEMPO (baseline) 300 684 4-hydroxy-TEMPO (baseline) 300 7364-oxo-TEMPO/2,5-di-t-butylhydroquinone 300/600 1974-oxo-TEMPO/2,5-di-t-butyl-1,4-benzoquinone 300/600 1804-oxo-TEMPO/2,5-di-t-butylhydroquinone/2,5-di-t-butyl-1,4- 300/150/450112 benzoquinone4-oxo-TEMPO/2,5-di-t-butylhydroquinone/2,5-di-t-butyl-1,4- 300/60/540128 benzoquinone

TABLE 4 Performance of Hydrogen Donor Systems Using the Multi-Pass TestMethod Average Polymer Inhibitor System/Pass Make (wt %) M_(w) ofPolymer 300 ppm 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy (4-oxo-TEMPO)Pass 1 0.052 3,910 Pass 2 1.45 17,000 Pass 3 7.45 31,700 45 ppm4-oxo-TEMPO; 420 ppm PDA; 900 ppm DNBP; 3 cc/min air Pass 1 0.026 1,430Pass 2 0.150 1,330 Pass 3 0.363 1,760 48 ppm 4-oxo-TEMPO; 1125 ppm DNBPPass 1 0.146 3,840 Pass 2 0.485 4,340 Pass 3 0.640 4,120PDA = N-phenyl-N′-(1,4-dimethylpentyl)-para-phenylenediamineDNBP = 2,4-dinitro-6-sec-butylphenol

In view of the many changes and modifications that can be made withoutdeparting from principles underlying the invention, reference should bemade to the appended claims for an understanding of the scope of theprotection to be afforded the invention.

1. A method for inhibiting the premature polymerization and the polymergrowth of ethylenically unsaturated monomers comprising adding to saidmonomers an effective amount of at least one inhibitor that is ahydrogen donor selected from the group consisting of: (A) a compound ofthe structure

wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers, and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members; (B) a compound of the structure

wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers, and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members; (C) a compound of the structure

wherein R₁₁₃, R₁₁₄, and R₁₁₅ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclicmoieties; and (D) a compound of the structure

wherein R₁₁₆, R₁₁₇, R₁₁₈, R₁₁₉, R₁₂₀, R₁₂₁, R₁₂₂, and R₁₂₃ areindependently selected from the group consisting of hydrogen, alkyl,aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl,OR₁₁₀, NR₁₁₀R₁₁₁, SR₁₁₀, NO₂, NO, CN, COR₁₁₂, halogen, and/or any twoadjacent groups can be taken together to form ring structure(s) of fiveto seven members, R₁₁₀ and R₁₁₁ are independently selected from thegroup consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic,substituted alkyl or aryl where the substituents are C, O, N, S, or P,and COR₁₀₂ or R₁₁₀ and R₁₁₁ can be taken together to form a ringstructure of five to seven members, R₁₁₂ is R₁₀₂, OR₁₀₂, or NR₁₀₂R₁₀₃,and R₁₀₂ and R₁₀₃ are independently selected from the group consistingof hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₂ andR₁₀₃ can be taken together to form a ring structure of five to sevenmembers.
 2. The method of claim 1 wherein the inhibitor is of thestructure

wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers; and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members.
 3. The method of claim 1 wherein the inhibitor is ofthe structure

wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers; and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members.
 4. The method of claim 1 wherein the inhibitor is ofthe structure

wherein R₁₁₃, R₁₁₄, and R₁₁₅ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclicmoieties.
 5. The method of claim 1 wherein the inhibitor is of thestructure

wherein R₁₁₆, R₁₁₇, R₁₁₈, R₁₁₉, R₁₂₀, R₁₂₁, R₁₂₂, and R₁₂₃ areindependently selected from the group consisting of hydrogen, alkyl,aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl,OR₁₁₀, NR₁₁₀R₁₁₁, SR₁₁₀, NO₂, NO, CN, COR₁₁₂, halogen, and/or any twoadjacent groups can be taken together to form ring structure(s) of fiveto seven members; R₁₁₀ and R₁₁₁ are independently selected from thegroup consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic,substituted alkyl or aryl where the substituents are C, O, N, S, or P,and COR₁₀₂ or R₁₁₀ and R₁₁₁ can be taken together to form a ringstructure of five to seven members; R₁₁₂ is R₁₀₂, OR₁₀₂, or NR₁₀₂R₁₀₃;and R₁₀₂ and R₁₀₃ are independently selected from the group consistingof hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₂ andR₁₀₃ can be taken together to form a ring structure of five to sevenmembers.
 6. The method of claim 1 wherein the inhibitor is selected fromthe group consisting of diethylhydroxylamine, cyclohexanoneoxime,dibenzylhydroxylamine, 2,4-dinitro-6-sec-butylphenol,N-phenyl-N′-(1,4-dimethylpentyl)-para-phenylenediamine,2,5-di-t-butylhydroquinone, 2,5-di-t-amylhydroquinone,methylhydroquinone, 4-t-butylhydroquinone, 4-t-butylcatechol,octanethiol, 2,6-di-t-butyl-4-ethylphenol/Cu(I)naphthenate,dihydroanthracene, N-t-butyl-2-benzothiazole-sulfenamide, andN-methyl-4-nitroaniline.
 7. The method of claim 1 wherein a transitionmetal is added.
 8. The method of claim 7 wherein the transition metal iscopper.
 9. A method for inhibiting the premature polymerization and thepolymer growth of ethylenically unsaturated monomers comprising addingto said monomers (A) at least one first inhibitor that is a hydrogendonor selected from the group consisting of: (1) a compound of thestructure

 wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers, and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members; (2) a compound of the structure

 wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers, and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members; (3) a compound of the structure

 wherein R₁₁₃, R₁₁₄, and R₁₁₅ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclicmoieties; and (4) a compound of the structure

 wherein R₁₁₆, R₁₁₇, R₁₁₈, R₁₁₉, R₁₂₀, R₁₂₁, R₁₂₂, and R₁₂₃ areindependently selected from the group consisting of hydrogen, alkyl,aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl,OR₁₁₀, NR₁₁₀R₁₁₁, SR₁₁₀, NO₂, NO, CN, COR₁₁₂, halogen, and/or any twoadjacent groups can be taken together to form ring structure(s) of fiveto seven members, R₁₁₀ and R₁₁₁ are independently selected from thegroup consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic,substituted alkyl or aryl where the substituents are C, O, N, S, or P,and COR₁₀₂ or R₁₁₀ and R₁₁₁ can be taken together to form a ringstructure of five to seven members, R₁₁₂ is R₁₀₂, OR₁₀₂, or NR₁₀₂R₁₀₃,and R₁₀₂ and R₁₀₃ are independently selected from the group consistingof hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₂ andR₁₀₃ can be taken together to form a ring structure of five to sevenmembers; and (B) at least one second inhibitor having the followingstructural formula:

 wherein R₁ and R₄ are independently selected from the group consistingof hydrogen, alkyl, and heteroatom-substituted alkyl, R₂ and R₃ areindependently selected from the group consisting of alkyl andheteroatom-substituted alkyl, and X₁ and X₂ (1) are independentlyselected from the group consisting of halogen, cyano, amido, —S—C₆H₅,carbonyl, alkenyl, alkyl of 1 to 15 carbon atoms, COOR₇, —S—COR₇, and—OCOR₇, wherein R₇ is alkyl or aryl, or (2) taken together, form a ringstructure with the nitrogen.
 10. The method of claim 9 herein the firstinhibitor is of the structure

wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers; and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members.
 11. The method of claim 9 wherein the first inhibitoris of the structure

wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers; and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members.
 12. The method of claim 9 wherein the first inhibitoris of the structure

wherein R₁₁₃, R₁₁₄, and R₁₁₅ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclicmoieties.
 13. The method of claim 9 wherein the first inhibitor is ofthe structure

wherein R₁₁₆, R₁₁₇, R₁₁₈, R₁₁₉, R₁₂₀, R₁₂₁, R₁₂₂, and R₁₂₃ areindependently selected from the group consisting of hydrogen, alkyl,aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl,OR₁₁₀, NR₁₁₀R₁₁₁, SR₁₁₀, NO₂, NO, CN, COR₁₁₂, halogen, and/or any twoadjacent groups can be taken together to form ring structure(s) of fiveto seven members; R₁₁₀ and R₁₁₁ are independently selected from thegroup consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic,substituted alkyl or aryl where the substituents are C, O, N, S, or P,and COR₁₀₂ or R₁₁₀ and R₁₁₁ can be taken together to form a ringstructure of five to seven members; R₁₁₂ is R₁₀₂,OR₁₀₂, or NR₁₀₂R₁₀₃;and R₁₀₂ and R₁₀₃ are independently selected from the group consistingof hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₂ andR₁₀₃ can be taken together to form a ring structure of five to sevenmembers.
 14. The method of claim 9 wherein the first inhibitor isselected from the group consisting of diethylhydroxylamine,cyclohexanoneoxime, dibenzylhydroxylamine,2,4-dinitro-6-sec-butylphenol,N-phenyl-N′-(1,4-dimethylpentyl)-para-phenylenediamine,2,5-di-t-butylhydroquinone, 2,5-di-t-amylhydroquinone,methylhydroquinone, 4-t-butylhydroquinone, 4-t-butylcatechol,octanethiol, 2,6-di-t-butyl-4-ethylphenol/Cu(I)naphthenate,dihydroanthracene, N-t-butyl-2-benzothiazole-sulfenamide, andN-methyl-4-nitroaniline.
 15. The method of claim 9 wherein a transitionmetal is added.
 16. The method of claim 15 wherein the transition metalis copper.
 17. The method of claim 9 wherein the second inhibitor is ofthe structure

wherein R₁ and R₄ are independently selected from the group consistingof hydrogen, alkyl, and heteroatom-substituted alkyl and R₂ and R₃ areindependently selected from the group consisting of alkyl andheteroatom-substituted alkyl, and the

portion represents the atoms necessary to form a five-, six-, orseven-membered heterocyclic ring.
 18. The method of claim 17 wherein thesecond inhibitor contains one or more nitroxyls selected from the groupconsisting of: N,N-di-tert-butylnitroxide; N,N-di-tert-amylnitroxide;N-tert-butyl-2-methyl-1-phenyl-propylnitroxide;N-tert-butyl-1-diethylphosphono-2,2-dimethylpropylnitroxide;2,2,6,6-tetramethyl-piperidinyloxy;4-amino-2,2,6,6-tetramethyl-piperidinyloxy;4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;4-oxo-2,2,6,6-tetramethyl-piperidinyloxy;4-dimethylamino-2,2,6,6-tetramethyl-piperidinyloxy;4-ethanoyloxy-2,2,6,6-tetramethyl-piperidinyloxy;2,2,5,5-tetramethylpyrrolidinyloxy;3-amino-2,2,5,5-tetramethylpyrrolidinyloxy;2,2,4,4-tetramethyl-1-oxa-3-azacyclopentyl-3-oxy;2,2,4,4-tetramethyl-1-oxa-3-pyrrolinyl-1-oxy-3-carboxylic acid;2,2,3,3,5,5,6,6-octamethyl-1,4-diazacyclohexyl-1,4-dioxy;4-bromo-2,2,6,6-tetramethyl-piperidinyloxy;4-chloro-2,2,6,6-tetramethyl-piperidinyloxy;4-iodo-2,2,6,6-tetramethyl-piperidinyloxy;4-fluoro-2,2,6,6-tetramethyl-piperidinyloxy;4-cyano-2,2,6,6-tetramethyl-piperidinyloxy;4-carboxy-2,2,6,6-tetramethyl-piperidinyloxy;4-carbomethoxy-2,2,6,6-tetramethyl-piperidinyloxy;4-carbethoxy-2,2,6,6-tetramethyl-piperidinyloxy;4-cyano-4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;4-methyl-2,2,6,6-tetramethyl-piperidinyloxy;4-carbethoxy-4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;4-hydroxy-4-(1-hydroxypropyl)-2,2,6,6-tetramethyl-piperidinyloxy;4-methyl-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;4-carboxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;4-carbomethoxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;4-carbethoxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;4-amino-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;4-amido-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;3,4-diketo-2,2,5,5-tetramethylpyrrolidinyloxy;3-keto-4-oximino-2,2,5,5-tetramethylpyrrolidinyloxy;3-keto-4-benzylidine-2,2,5,5-tetramethylpyrrolidinyloxy;3-keto-4,4-dibromo-2,2,5,5-tetramethylpyrrolidinyloxy;2,2,3,3,5,5-hexamethylpyrrolidinyloxy;3-carboximido-2,2,5,5-tetramethylpyrrolidinyloxy;3-oximino-2,2,5,5-tetramethylpyrrolidinyloxy;3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;3-cyano-3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;3-carbomethoxy-3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;3-carbethoxy-3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;2,2,5,5-tetramethyl-3-carboxamido-2,5-dihydropyrrole-1-oxyl;2,2,5,5-tetramethyl-3-amino-2,5-dihydropyrrole-1-oxyl;2,2,5,5-tetramethyl-3-carbethoxy-2,5-dihydropyrrole-1-oxyl;2,2,5,5-tetramethyl-3-cyano-2,5-dihydropyrrole-1-oxyl;bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)succinate;bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipate;bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)sebacate;bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)n-butylmalonate;bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)phthalate;bis(1-oxy-2,2,6,6-tetramethylpiperidin-4-yl)isophthalate;bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)terephthalate;bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)hexahydroterephthalate;N,N′-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipamide;N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-caprolactam;N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-dodecylsuccinimide;2,4,6-tris-[N-butyl-N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)]-s-triazine;and 4,4′-ethylenebis(1-oxyl-2,2,6,6-tetramethylpiperazin-3-one).
 19. Acomposition comprising: (A) at least one first inhibitor that is ahydrogen donor selected from the group consisting of: (1) a compound ofthe structure

 wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers, and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members; (2) a compound of the structure

 wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₀ andR₁₀₁ can be taken together to form a ring structure of five to sevenmembers, and R₁₀₂ and R₁₀₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or R₁₀₂ and R₁₀₃ can be taken together to form a ring structure of fiveto seven members; (3) a compound of the structure

 wherein R₁₁₃, R₁₁₄, and R₁₁₅ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclicmoieties; and (4) a compound of the structure

 wherein R₁₁₆, R₁₁₇, R₁₈, R₁₉, R₁₂₀, R₁₂₁, R₁₂₂, and R₁₂₃ areindependently selected from the group consisting of hydrogen, alkyl,aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl,OR₁₁₀, NR₁₁₀R₁₁₁, SR₁₁₀, NO₂, NO, CN, COR₁₁₂, halogen, and/or any twoadjacent groups can be taken together to form ring structure(s) of fiveto seven members, R₁₁₀ and R₁₁₁, are independently selected from thegroup consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic,substituted alkyl or aryl where the substituents are C, O, N, S, or P,and COR₁₀₂ or R₁₁₀ and R₁₁₁ can be taken together to form a ringstructure of five to seven members, R₁₁₂ is R₁₀₂, OR₁₀₂, or NR₁₀₂R₁₀₃,and R₁₀₂ and R₁₀₃ are independently selected from the group consistingof hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₂ andR₁₀₃ can be taken together to form a ring structure of five to sevenmembers; and (B) at least one second inhibitor having the followingstructural formula:

 wherein R₁ and R₄ are independently selected from the group consistingof hydrogen, alkyl, and heteroatom-substituted alkyl, R₂ and R₃ areindependently selected from the group consisting of alkyl andheteroatom-substituted alkyl, and X, and X₂ (1) are independentlyselected from the group consisting of halogen, cyano, amido, —S—C₆H₅,carbonyl, alkenyl, alkyl of 1 to 15 carbon atoms, COOR₇, —S—COR₇, and—OCOR₇, wherein R₇ is alkyl or aryl, or (2) taken together, form a ringstructure with the nitrogen.
 20. The composition of claim 19 wherein thesecond inhibitor is of the structure

wherein R₁ and R₄ are independently selected from the group consistingof hydrogen, alkyl, and heteroatom-substituted alkyl and R₂ and R₃ areindependently selected from the group consisting of alkyl andheteroatom-substituted alkyl, and the

portion represents the atoms necessary to form a five-, six-, orseven-membered heterocyclic ring.